1. Field of the Invention
The present invention relates to a plug valve which a fluid flows through or is interrupted by. The plug valve is capable of reliably preventing the fluid from leakage during the interruption.
2. Description of the Related Art
Conventionally, a plug valve comprises a main valve body and a plug. The valve body is provided with a first passage and a second passage having their axes coincident with each other, and includes a columnar opening disposed between the first passage and the second passage. The plug is rotatably attached to the opening.
The plug valve has a through-hole penetrating therethrough in a direction substantially coincident with the axial direction of the first and second passages. Accordingly, the plug is constructed such that the fluid flows from the first passage through the through-hole to the second passage.
The plug is rotatable from a communication position at which the through-hole provided in the opening of the main valve body communicates with the first passage and the second passage, to an interruption position at which the fluid is prevented from flowing from the first passage to the second passage. In this arrangement, a seal member is provided on the periphery of the through-hole of the plug. The seal member constitutes a seal mechanism which prevents the fluid from flowing from the first passage to the second passage when the plug is rotated from the communication position to the interruption position (see, for example, Japanese Laid-Open Patent Publication No. 5-215250).
However, when the seal member crosses the first passage in the rotation of the plug, the seal member projects or floats over the seal groove due to the difference in pressure of the fluid between the first passage and the second passage. If the plug is rotated while the seal member projects, the seal member is caught between the end surface of the hole of the first passage and the end surface of the seal groove. When the seal member is damaged, the fluid leaks and sufficient seal function is not effected.